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The electron density topology of carbon monoxide (CO) on dry and hydrated platinum is evaluated under the quantum theory of atoms in molecules (QTAIM) and by adsorbate orbital approaches. The impact of water co-adsorbate on the electronic, structural, and vibrational properties of CO on Pt are modelled by periodic density functional theory (DFT). At low CO coverage, increased hydration weakens C–O bonds and strengthens C–Pt bonds, as verified by changes in bond lengths and stretching frequencies. These results are consistent with QTAIM, the 5σ donation-2π* backdonation model, and our extended π-attraction σ-repulsion model (extended π-σ model). This work links changes in the non-zero eigenvalues of the electron density Hessian at QTAIM bond critical points to changes in the π and σ C–O bonds with systematic variation of CO/H2O co-adsorbate scenarios. QTAIM invariably shows bond strengths and lengths as being negatively correlated. For atop CO on hydrated Pt, QTAIM and phenomenological models are consistent with a direct correlation between C–O bond strength and CO coverage. However, DFT modelling in the absence of hydration shows that C–O bond lengths are not negatively correlated to their stretching frequencies, in contrast to the Badger rule: When QTAIM and phenomenological models do not agree, the use of the non-zero eigenvalues of the electron density Hessian as inputs to the phenomenological models, aligns them with QTAIM. The C–O and C–Pt bond strengths of bridge and three-fold bound CO on dry and hydrated platinum are also evaluated by QTAIM and adsorbate orbital analyses. [ABSTRACT FROM AUTHOR]

Laser-activated membrane introduction mass spectrometry (LAMIMS), a high-throughput screening method, evaluates heterogeneous catalysts under realistic reactor conditions. It is a precise, versatile system requiring no moving parts. The catalyst array is supported on carbon paper overlaid upon a silicone rubber membrane configured in a variation of membrane introduction mass spectrometry as introduced by Cooks. The carbon paper serves as a heat-dissipating gas diffusion layer that permits laser heating of catalyst samples far above the decomposition temperature of the polymer membrane that separates the array from the mass spectrometer vacuum chamber. A computer-controlled C[O.sub.2] bar code writing laser is used for fine-tune heating of the catalyst spots above the base temperature of the LAMIMS reactor. The detailed design and performance of LAMIMS is demonstrated on arrays of 'real world' bulk water--gas shift catalysts using natural and isotopically labeled reactor feed streams. A bulk catalyst array spot can be evaluated for activity and selectivity in as little as 1.5 min. All array screening results were confirmed by industrial microreactor evaluations.

Density functional theory (DFT) calculated spectra was used to correlate the operando infrared (IR) spectroscopy, polarization modulated IR spectroscopy (PM-IRRAS) of Pt-Nafion interfaces, and attenuated total reflectance IR spectroscopy of bulk Nafion to understand the Nafion functional group coadsorption responsible for the Stark tuning of [CO.sub.ads] on high surface area fuel cell electrodes. A model of the Nafion-Pt interface with appropriate dihedral and native bond angles which was consistent with experimental and calculated spectra indicated direct adsorption of the C[F.sub.3] and S[O.sub.3.sup.-] functional groups on Pt.

Electron spin resonance (ESR) is used for monitoring the local environment of 2,2,6,6-tetramethyl-4-piperidone N-oxide (Tempone) spin probe in water and methanol mixtures in solution and in [Li.sup.+] ion exchanged Nafion 117 membranes. The probe has displayed significant local ordering in the aqueous phase of Nafion membranes that is diminished with increasing methanol concentration.

The potential-dependent C[O.sub.2] exhaust from the cathode of an operating direct methanol fuel cell (DMFC) is monitored by online electrochemical mass spectrometry (ECMS) with air and with [H.sub.2] at the cathode. Electron spin resonance (ESR) measurements of the rotational diffusion rate of a nitroxide spin probe in Nafion have provided information concerning the effect of the [C.sub.MeOH] on the local state of the polymer.

The in situ X-ray absorption spectroscopy, ex situ X-ray fluorescence and X-ray powder diffraction have helped in the analysis of phase segregated nanostructured PtRu anode catalysts in an operating direct methanol fuel cell. The extended X-ray absorption fine structure analysis has showed that the Ru-O bond lengths are shorter than those reported for Ru-O of ruthenium oxides, indicating that the phases in which the Ru resides in the catalysts are not similar to oxides.

(super 195)Pt NMR spectroscopic and electrochemical measurements are carried out on commercial Pt-Ru alloy nanoparticle samples to investigate the effect of high temperature annealing in different vacuum/gas-phase environments. The results suggest that optimizing the amount of surface Ru by heat treatment at temperatures near 200 degree Celsius, in a hydrogen atmosphere, is utilized to produce Pt-Ru alloy nanoparticles with high methanol oxidation activity.

ABSTRACT The time-dependent IR spectra during dehydration of fully hydrated Nafion show the reversible disappearance of the 1061 cm−1 and 969 cm−1 concurrent with the emergence of peaks at ∼928 cm−1 and ∼1408 cm−1. The first pair of group modes is associated with a dissociated exchange group (sulfonate) with a local C3V symmetry. The C3V group modes shift with state-of-hydration: The 969 cm−1 peak completely vanishes and the 1061 cm−1 is reduced to a small shoulder at 1070 cm−1 at end of dehydration. The C3V group modes are replaced by the pair of group modes of an associated exchange group (sulfonic acid) with C1 local symmetry. The density functional theory normal mode analysis confirms that the sulfonic acid/sulfonate site plays a dominant role in the C1 and C3V group modes, respectively. This work clarifies the importance of assigning fluoropolymers peaks as group modes rather than traditional single functional group assignments as is often the case with the ∼1061 cm−1 and ∼969 cm−1 C3V group modes. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1329-1334 [ABSTRACT FROM AUTHOR]

Combinatorial methods provide a means for accelerating the discovery of fuel cell catalysts. The first example of parallel fuel cell catalysts screening was an indirect method that used fluorescent chemosensors to detect changes in pH in proximity to electrocatalyst spots. Serial direct electrochemical methods have been developed that use voltammetry, chronoamperometry, and scanning electrochemical microscopy. An array fuel cell screens catalysts simultaneously, using high-performance fuel cell components. Heuristic models based on mechanistic and spectroscopic studies provide guidance for library development, and detailed studies of discovered catalysts can help to refine these models. The remaining challenges are the development of high throughput synthetic methods that can enable the use of discovery level and focus level screening. Until these synthetic methods are developed, a greater emphasis should be placed on smaller libraries with design of experiment strategies leveraged with informatics and data mining. [ABSTRACT FROM AUTHOR]

An in situ x-ray absorption spectroscopy cell was designed to study the structural and electronic properties of high performance polymer electrolyte fuel cell electrocatalysts. The cell was operated as a standard fuel cell and x-ray absorption data were collected simultaneously as the fuel cell operating conditions were varied. The cell was used to examine platinum-ruthenium alloy electrocatalysts incorporated in the anode of a fuel cell membrane and electrode assembly. In situ Pt L[sub III] and Ru K edge x-ray absorption near-edge structure (XANES) data were collected at five selected current densities of the operating fuel cell. The XANES data show that under normal fuel cell operating conditions, the metallic nature of the Pt-Ru catalyst is retained. The cell design enabled an in situ fuel cell study at a synchrotron source in which no additional electrolytes were required. [ABSTRACT FROM AUTHOR]

Density functional calculations (DFT) on carbon monoxide (CO) adsorbed on platinum, platinum-osmium, and platinum-ruthenium-osmium nanoclusters are used to elucidate changes on the adsorbate internal bond and the carbon-metal bond, as platinum is alloyed with osmium and ruthenium atoms. The relative strengths of the adsorbate internal bond and the carbon-metal bond upon alloying, which are related to the DFT calculated C-O and C-Pt stretching frequencies, respectively, cannot be explained by the traditional 5σ-donation/2π*-back-donation theoretical model. Using a modified π-attraction σ-repulsion mechanism, we ascribe the strength of the CO adsorbate internal bond to changes in the polarization of the adsorbate-substrate hybrid orbitals towards carbon. The strength of the carbon-metal bond is quantitatively related to the CO contribution to the adsorbate-substrate hybrid orbitals and the sp and d populations of adsorbing platinum atom. This work complements prior work on corresponding slabs using periodic DFT. Similarities and differences between cluster and periodic DFT calculations are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Metal hydride foils can serve as support structures for electronically insulating proton conductors to afford metal hydride-barrier composite electrolytes operating at temperatures in between molten carbonate fuel cells and phosphoric acid fuel cells. In addition, such barrier electrolytes entirely decouple anode and cathode reactant streams and water management, enabling general analytical methods that separate electrode kinetic from reactant crossover effects. This strategy was exemplified with a direct methanol fuel cell catalyzed with PtRu. No fuel-stream reactant or product crossover other than protons was detected. [Copyright &y& Elsevier]

Abstract: A polymer electrolyte fuel cell enables operando X-ray absorption and infrared spectroscopy of the membrane electrode assembly catalytic layer with flowing fuel and air streams at controlled temperature. Time-dependent X-ray absorption near edge structure spectra of the Pt and Ni edge of Pt based catalysts of an air-breathing cathode show that catalyst restructuring, after a potential step, has time constants from minutes to hours. The infrared Stark tuning plots of CO adsorbed on Pt at 100, 200, 300 and 400mV vs. hydrogen reference electrode were obtained. The Stark tuning plots of CO adsorbed at 400mV exhibit a precipitous drop in frequency coincident with the adsorption potential. The turn-down potential decreases relative to the adsorption potential and is approximately constant after 300mV. These Stark tuning characteristics are attributed to potential dependent adsorption site selection by CO and competitive adsorption processes. [Copyright &y& Elsevier]

Abstract: The electrochemical reduction of CO2 was studied on a copper mesh electrode in aqueous solutions containing 3M solutions of KCl, KBr and KI as the electrolytes in a two and three phase configurations. Electrochemical experiments were carried out in a laboratory-made, divided H-type cell. The working electrode was a copper mesh, while the counter and reference electrodes were Pt wire and Ag/AgCl electrode, respectively. Results of our work suggest a reaction mechanism for the electrochemical reduction of CO2 in the two phase configuration where the presence of Cu-X as the catalytic layer facilitates the electron transfer from the electrode to CO2. Electron-transfer to CO2 may occur via the X ad −(Br−, Cl−, I−)–C bond, which is formed by the electron flow from the specifically adsorbed halide anion to the vacant orbital of CO2. The stronger the adsorption of the halide anion to the electrode, the more strongly CO2 is restrained, resulting in higher CO2 reduction current. Furthermore, it is suggested that specifically adsorbed halide anions could suppress the adsorption of protons, leading to a higher hydrogen overvoltage. These effects may synergistically mitigate the overpotential necessary for CO2 reduction, and thus increase the rate of electrochemical CO2 reduction. [Copyright &y& Elsevier]